That's why the eye test can be unreliable. People have their own biases and misconceptions. If you think a player is bad, you will certainly focus more on the bad things he does and pass over the good. I know that stats don't tell the full story, but I'll take a long list of stats over an extended period over someone's eye test in most cases.
McFred can be improved upon, but they're not complete shit like some of our fans say. They would do well in any team outside of the top 5/6 clubs. Our main rivals have better midfielders. That's a problem. We need to upgrade, but only with the right players. McFred look quite good in games where we're having to defend more. They do well in that kind of midfield battle.
Lingard looked great for West Ham, because he's allowed space to run into. Half of the goals he scored were on the counter, he got a couple long shots outside of the box, and two pens. Well, he missed one pen, but scored the follow up. It's the kind of space that he doesn't get in most games for United. He will look good for any team that gets space to work with. That's not really a bad thing. You still have to be a good player to make use of the chances you get, but it's harder at bigger clubs that have to break down low blocks. You're also right that lower pressure at smaller clubs will help some players to perform better.
When we get chances to counter, we usually punish the opposition. Rashford is so deadly with space to run into.